The present invention relates to methods and apparatus for printing patterns on substrates. The invention is particularly useful for printing patterns (conductive or non-conductive) on panels in the manufacture of Electronics Components (ELC), Flat Panel Displays (FPD), various conductive and non-conductive patterns or holes on/in PCBs, masks for soldering, plating, etching or plasma etching and multi-layer printed circuit boards (PCBs). It is described below with respect to this last application merely for purposes of example.
PCBs (sometimes referred to as PWBs, or printed wiring boards) are widely used in electronic systems. Such PCBs may include as many as 20 or 30 layers, each having its own electrically conductive pattern layers as well as non-conductive layers, with various layers electrically connected together by interlayer connections, such as pins, posts, plated-through-holes (PTHs), pads and the like. Because of the interrelationship between the patterns in the various layers, a relatively high degree of resolution (i.e., the smallest incremental step in separating a measurement in its constituent parts) is required in printing the patterns; and because of the large number of layers in a PCB, a relatively high rate of throughput is required for a commercially usable method.
However, the throughput of a manufacturing process is inversely related in a non-linear manner to the required resolution. Thus, if the resolution is doubled, the time required for producing the respective layer is multiplied approximately by four. Therefore, the existing manufacturing processes for producing PCBs require engineering compromises that can significantly affect the resolution and/or the throughput.
An object of the present invention is to provide a method, and also an apparatus, for printing a pattern on a substrate enabling higher resolution, without unduly reducing throughput where high resolution is needed. Another object of the present invention is to provide a method and apparatus for printing patterns particularly useful in the production of multi-layer PCBs.
According to one aspect of the present invention, there is provided a method of printing a pattern on a substrate, comprising: applying a photoresist layer over the substrate; locating a laser direct-imaging exposure head with respect to the substrate to print the pattern on the substrate by the application of laser energy to the photoresist layer on the substrate while relative movement is effected between the exposure head and the substrate by a drive; defining a first portion of the substrate to be printed with a first portion of the pattern at a first resolution, and a second portion of the substrate to be printed with a second portion of the pattern at a second, higher resolution; and controlling the drive and the exposure head to cause the exposure head to print the first portion of the pattern on the first portion of the substrate at the first resolution, and the second portion of the pattern on the second portion of the substrate at the second, higher resolution.
In the preferred embodiments of the invention described below, the photoresist layer applied over the substrate is a bi-state photoresist material which assumes a non-exposed state up to a predetermined exposure threshold, an exposed state at the predetermined exposure threshold, and remains in the exposed state beyond the predetermined exposure threshold, being insensitive to further exposure beyond the predetermined exposure threshold.
According to further features in the described preferred embodiments, the second portion of the substrate to be printed at the second, higher resolution is defined by specifying the borders of the second portion of the pattern, while the remainder of the substrate is defined as the first portion of the substrate to be printed with the first portion of the pattern at the first resolution.
According to still further features in the described preferred embodiments, the drive and the exposure head are controlled such that smaller increments of movement are effected by the drive between the exposure head and the substrate while the exposure head is printing the second portion of the pattern at the second, higher resolution than while the exposure head is printing the first portion of the pattern at the first resolution.
In one described preferred embodiment, the drive and the exposure head are controlled to cause the exposure head to print the first portion of the pattern on the first portion of the substrate at the first resolution, and the second portion of the pattern on the second portion of the substrate at the second, higher resolution, both during a single scan of the exposure head with respect to the substrate.
In a second described preferred embodiment, the drive and the exposure head are controlled to cause the exposure head to print the first and second portions of the pattern on the first and second portions of the substrate at the first resolution during a first scan of the exposure head with respect to the substrate; and then to print only the second portion of the pattern on the second portion of the substrate at the second, higher resolution during a second scan of the exposure head with respect to the substrate.
According to further features in the described preferred embodiments, the drive includes a first motor and transmission which moves the substrate with respect to the exposure head along the longitudinal axis of the substrate, and a second motor and transmission which moves the exposure head with respect to the substrate along the transverse axis of the substrate; the first motor and transmission being controlled to move the substrate through smaller increments of distance when the exposure head is printing the second portion of the pattern at the second, higher resolution, than when printing the first portion of the pattern at the first resolution.
In one described preferred embodiment, the exposure head includes a linear array of lasers extending in the direction of the longitudinal axis of the substrate, the lasers being selectively controlled to print at the first resolution or the second higher resolution during the transverse movement of the exposure head with respect to the substrate.
In a second described embodiment, the exposure head includes a laser producing a laser beam, and a rotating polygonal mirror for reflecting the laser beam in the direction of the longitudinal axis; the laser beam being selectively controlled with respect to the mirror to print at the first resolution or the second, higher resolution during the transverse movement of the exposure head with respect to the substrate.
According to another aspect of the present invention, there is provided apparatus for printing a pattern on a photoresist layer carried by a substrate, comprising: a holder for carrying the substrate; a laser direct-imaging exposure head mounted with respect to the holder to print the pattern on the photoresist layer of the substrate when the substrate is carried by the holder; a drive for effecting relative movement between the exposure head and the substrate; and a controller programmed to enable: defining a first portion of the substrate to be printed with a first portion of the pattern at a first resolution, and a second portion of the substrate to be printed with a second portion of the pattern at a second, higher resolution; and controlling the drive and the exposure head to cause the exposure head to print the first portion of the pattern on the first portion of the substrate at the first resolution, and the second portion of the pattern on the second portion of the substrate at the second, higher resolution.
As will be described more particularly below, such a method and apparatus enable the printing of patterns on substrates at a relatively high resolution as and where needed without unduly reducing the throughput and thereby makes the method and apparatus particularly useful for producing multi-layer PCBs. Thus, in most cases, the PCBs include only relatively small portions of a pattern which require higher resolution than other portions, and/or only certain layers having portions requiring the higher resolution. By using the method of the present invention, the manufacturing process is slowed-down only with respect to those layer portions requiring the higher resolution, and therefore the overall throughput of the process is not reduced anywhere near the extent it would be if a complete layer, or all the layers of a PCB, were to be printed at the higher resolution.
While the preferred embodiment of the invention described below describes selecting two resolutions (e.g., a normal resolution and a higher resolution), it will be appreciated that the method and apparatus could also be used for selecting more than two resolutions (e.g., a normal resolution, a higher resolution, and an ultra-high resolution) according to the particular application, thereby enabling printing patterns at the optimum resolution for each pattern without unduly reducing the overall throughput of the process.
Further features and advantages of the invention will be apparent from the description below.